A device for fastening two pieces together under pressure includes a spring pressure device which has the advantage of apply a predetermined pressure that is essentially constant on the two pieces, and a connection device. For fastening the two pieces, it is necessary to assemble them correctly with the pressure device and the connection device in such a way that the pressure device can be compressed to exert the desired forces. It is understood that placement of the pressure device is especially difficult because the pieces are of small dimensions and the fastening pressure is elevated. In addition it is sometimes desirable that the fastener be removable. Clearly, the disassembly of a pressure fastening device causes the same difficulties as assembly does. Another difficulty is added when one of the pieces is subjected to conditions that can cause the fastening device to change states. For example, exposure to heat of at least one of the pieces can deform or corrode certain parts of the fastening device and make disassembly difficult. The removable pressure fastener of a heat sink on a high density integrated circuit package includes these difficulties and thus will be used as an example to illustrate the invention.
In effect while the size of integrated circuits does not stop increasing and is actually over one centimeter on a side the packages are designed to become smaller and smaller For example, in the packages such as those described in French patent applications No. 87 13416 and 87 13417 of the applicant, the inner plate for drawing off heat is two to three times as large as that of the integrated circuit, which would be about 25 mm on a side. Fastening the heat sink is usually done on a lateral column of the plate. Various forms that permit removal have already been considered, notably gluing and screwing.
However, the glue adds thermal resistance to the heat evacuation path and gluing is a delicate operation. As to screwing, it is difficult with such small pieces and causes difficulty and higher costs for the uniform pressure contact required of the heat sink against the liner plate of the circuit package. In addition, heating the two pieces may make unfastening difficult after a certain period of time. A pressure fastening device could be advantageous to assure uniform contact at a predetermined pressure of the heat sink against the box inner plate, but it has previously been seen that a fastening device of this type simultaneously complicates the structure and the operations of assembly and disassembly.
Another approach for cooling integrated circuits consists of applying one and the same heat sink on a group of integrated circuits mounted on a printed circuit card. The heat sink is still fixed on the solid posts of the card in such a way that it can be removed. It is also desirable in this case to have simple and efficient fastening devices which remain easily removable whatever the usage conditions of the pieces and their changes over time.